Apple //c and IIc Plus Firmware Banking

All Apple //c and IIc Plus computers except ROM $00 ship with two 16K firmware banks, each visible from $C100-$FFFF. The visible bank is switched by a write to $C028. This is not well-documented by the Apple documentation.

It is not possible to know which firmware bank is active except by reading locations in the firmware, there is no status bit available in the I/O space.

The ROM $00 machines may be upgraded to use a 32K ROM via a pair of solder/butterfly jumpers on the board.

The aux bank mostly contains firmware supporting the features not present or less mature in the original Apple //c: 3.5" drive/SmartPort, serial and keyboard buffering, memory expansion card, diagnostics, and (for the Plus) accelerator.